Recently, with interest in digital televisions and high-quality wide-screen televisions increasing, development of cathode ray tubes (CRTs), liquid crystal displays (LCDs), and plasma display panels (PDPs) is actively progressing.
CRTs, which are widely used as displays in TVs, are superior in resolution and image quality. However, they are not suitable for wide screens of 40 inches or larger because of size and weight.
LCDs are superior in terms of power consumption and driving voltage, but they are difficult to make into wide screens and are limited in view angle.
PDPs can be made into wide screens of about 60 inches. When manufacturing a PDP device, a partitioned barrier is formed on a bottom plate and phosphor layers of red, green, and blue are formed on the barrier. The bottom plate is then covered by a top plate, so that electrodes of the bottom plate and those of the top plate face each other in parallel. Then, discharge gas is injected therein and the device is sealed. Light radiated from plasma when the gas is discharged is used to transfer images, and the barrier separates each unit cell.
PDP devices are classified into direct current (DC) and alternating current (AC) types. In DC devices, electrodes are exposed to the discharge area, and in AC devices, a dielectric glass layer is formed on electrodes.
Referring to FIG. 1, a general PDP comprises a case 11, a cover 15 covering said case 11, a drive circuit board 12 located in said case 11, a panel assembly 13, and an electro-magnetic shielding filter 14. Said electro-magnetic shielding filter 14 is coated with a conducting film made of a conducting material, which offers a grounding path passing through the cover 15 to the case 11, and is placed on a glass plate. That is, an electromagnetic wave generated from the panel assembly 13 is passed through the conducting film of the electro-magnetic shielding filter 14 to the cover 15 and the case 11 before it reaches the user. For this purpose, the conducting film of the electro-magnetic shielding filter 14 is electrically connected to the cover 15 and the case 11.
In addition to shielding electromagnetic waves, said electro-magnetic shielding filter shields near infrared rays, protects the panel from external impact, improves color resolution, prevents reflection, and enhances contrast.
Formerly, transparent conducting films satisfying industrial class A (surface resistance <2.5Ω/□) standard, such as an Ag multi-deposited film, were widely used to manufacture electro-magnetic shielding filter films for plasma display panels. But recently, with the increase in demand for plasma display panels for families, copper etching meshes or conducting fiber meshes that satisfy the class B (surface resistance <1.5Ω/□) standard in which copper or nickel is coated on polyester fiber by electroless plating are being widely used.
Generally, mesh-type electro-magnetic shielding films are manufactured by applying copper foil having a thickness of about 10 μm on a polyethylene terephthalate substrate using a transparent adhesive and etching it to pitch ˜300 μm and linewidth ˜10 μm.
When this electro-magnetic shielding film is laminated with a functional film such as an antireflection film, an aperture is formed between the two films due to the copper foil mesh. Then, because of the difference in refractive indices of air and the polymer layer, light emergent from the plasma display panel becomes scattered, thereby reducing visibility. Therefore, the aperture should be removed by a process called transparentization.
For transparentization, a method of using a hot melt resin (Japan Patent Publication No. Hei 9-258378), a method of using adhesive, a method of filling a fluid resin (U.S. Pat. No. 6,197,408), and so forth have been developed. Among these, the most simple and widely used method is the method of using adhesive by which transparentization can be attained by applying pressure without an additional process. In transparentization using adhesive, the durability of the electro-magnetic shielding filter largely depends on the component of the adhesive.
In general, rubber, acryl, and silicone are widely used for adhesives. Among these, acrylic adhesives are the most popular for preparing functional adhesive compositions because of various melt properties. General acrylic adhesives have superior adhesivity when pressurized slightly at normal temperature. This is because polymer molecules comprising the adhesives are fluid and sensitive to pressure.
The electro-magnetic shielding filter offers colors with good resolution and image quality because of each functional film. If the filter discolors during use, it cannot offer good resolution and image quality. Therefore, there should be no change in transmittivity and color between before and after a durability test. Especially, color change due to oxidation of the copper mesh under high temperature and high humidity is a serious problem.
The durability of adhesive at the area where it contacts the copper mesh is a problem. Copper looks red in a neutral state, but it turns blue because of Cu2+ when oxidized. Therefore, the cause of discoloration of the transparentized adhesive is oxidation of copper.
In widely used acrylic adhesive compositions, several functional groups are used to crosslink the adhesive. Particularly, when monomers having acidic groups (—COON) are used, the acidic groups (—COON) oxidize copper under high temperature and high humidity, thereby discoloring the electro-magnetic shielding filter and reducing color resolution.
*Description of marks for a main part of drawings*11case12drive circuit board13panel assembly14electro-magnetic shielding filter15cover21, 31transparent substrate22, 32adhesive layer23, 33layers of copper foil24, 34black screen25, 35adhesive26, 36transparent substrate27, 37reinforcing glass38Color correction layer39near infrared ray shielding layer40near infrared ray shielding layer